This invention relates to a sheet stacking apparatus, in particular a spiral slot stacker, and to a method for controlling the feed of sheet material into slots of a continuously or intermittently rotating stacking wheel.
Spiral slot stackers are used for example in sheet testing and sorting apparatuses in which stack of sheets, for example bundles of bank notes, are first singled, subsequently guided through a sensor system for testing purposes and finally stacked in different stacks by means of spiral slot stackers. The function of the spiral slot stackers is to slow down the arriving single sheets by deflecting them into a spiral-shaped path before their final stacking. In most cases of application it is uncritical if not every slot of the stacking wheel receives a sheet or a slot receives more than one sheet by way of exception during stacking.
However, one must make sure the sheets do not collide with a partition separating the slots at the moment of its delivery to the stacking wheel. The leading edge of a sheet should therefore be supplied to the stacking wheel at an ideal input point between two partitions in order to ensure collision-free and complete feed of the sheet into a slot. Since the sheets are not always fed into the stacking wheel at a synchronous time interval due to slip in the transport system or due to different sheet formats, the problem of exact feed control poses itself regardless of whether the stacking wheel rotates intermittently or continuously. In both cases it is necessary to synchronize an asynchronous sheet feed and the rotating stacking wheel in such a way that each sheet is delivered into a slot of the stacking wheel completely and without collision.
In DE 27 56 223 C2 it is proposed that one determine quantitatively the deviation of the individual sheet from its ideal position by means of a sensor at a given constant rotating speed of the stacking wheel and urge the leading sheet edge so far down by an amount proportional to the determined deviation by means of a finger at the moment of sheet delivery that the leading sheet edge is fed into the slot at the ideal sheet input point. Quantitative measurement of the position deviation is done at a distance before the delivery point so that sufficient time is available for influencing the leading sheet edge by individually deflecting the finger.
In GB 2 168 687 A and EP 0 082 195 B1 it is proposed that one influence not the leading sheet edge but the positioning of the stacking wheel by first detecting the leading edge of an approaching sheet at a certain distance before the delivery point and thereupon influencing the step speed of the stacking wheel briefly in dependence on the transport speed of the sheet such that the leading sheet edge is fed into a slot of the stacking wheel at the ideal input point.
One disadvantage of the latter proposed solution is that with sheets in close succession very high accelerations of the stacking wheel are necessary in order to bring the next slot into the ideal feed position in time. Another disadvantage results in connection with overlapping sheets, occurring in particular in the processing of used bank notes due to their poor condition. In such cases there is a high probability that the trailing sheet will be incompletely grasped and thrown out of the stacking wheel.
The problem of the present invention is therefore to provide a sheet stacking apparatus and a method for controlling the feed of sheet material into slots of a stacking wheel, the stacking wheel being influenced so as to permit defined stacking even with very short sheet distances or overlapping sheets.
This problem is solved according to the invention by a method and sheet stacking apparatus with the features of the independent patent claims. Advantageous embodiments of the invention are stated in claims dependent thereon.
While in the prior art the approach of a leading sheet edge was determined and evaluated in order to use the determined data to take steps suitable for ensuring collision-free feed of the sheet into a slot, the invention provides for a group or groups of at least two sheets also to be detected by sensor technology and evaluated and for the evaluation result for this group or groups of sheets to be used to take steps suitable for ensuring reliable delivery of all sheets of said group into the slots of the stacking wheel. Since the evaluation takes account of not only the next approaching sheet but at least the next two approaching sheets, it is possible to control the feed of sheets into the stacking wheel prospectively for the total group of sheets. In particular, the kinematics of the stacking wheel can be influenced prospectively, said influence relating to the position and/or speed and/or acceleration of the stacking wheel.
For this purpose a sheet sensor is provided which determines one or more pieces of information about the approaching sheet material, for example, the distance between two sheets, the length of a sheet or the total length of a plurality of overlapping sheets, the total thickness of a plurality of overlapping sheets, or other information permitting conclusions on the relative position of two or more sheets.
The sheet sensor is expediently disposed at a sufficient distance before the stacking wheel so that the information about the following sheet relative to the preceding sheet can be evaluated and the stacking wheel accordingly influenced before the leading sheet begins to be fed into a slot of the stacking wheel. The distance between the stacking wheel and the sheet sensor should therefore correspond to a length composed of the maximum length of the sheets to be processed, the normal distance between the sheets and an additional path, the additional path being dimensioned in dependence on the transport speed such that sufficient time is available for evaluating the sheet sensor information and suitably influencing the individual sheet flow.
A preferred embodiment of the invention provides for combining the distance measurement between two sheets and the total length measurement of the sheets or overlapping sheets. This can be done in a simple manner with a single sheet sensor, which can be formed for example as a light barrier and is preferably located at the above-described distance from the stacking wheel. By means of the light barrier one can easily ascertain the presence of sheet material in the transport path. The time period passing between two consecutive sheets serves as a measure of the distance between the sheets, and the time period passing between the distance measurements as a measure of the length of a sheet or group of sheets. If the determined distance and/or length measure deviates impermissibly from a given threshold value, one selectively deviates from a given motion sequence of the stacking wheel synchronized with the sheet singling rate and influences the stacking wheel in accordance with the individually determined sheet flow by accelerating, slowing down or stopping the stacking wheel or rotating it at very low speed.
Depending on the type of ascertained irregularity one might take the following steps for example. If a distance between two sheets is ascertained which is below a minimum distance, the stacking wheel can be briefly stopped or rotated at very low rotating speed so that both sheets can be fed into a common slot. Alternatively, the rotating speed of the stacking wheel can be briefly increased in order to compensate the shortened distance so that the two sheets are fed into separate slots.
If the determined distance exceeds a given maximum distance, it is recommendable to slow down the stacking wheel briefly in order to take the enlarged distance into account so that the two sheets are reliably fed into separate slots.
If the determined length of a sheet or group of overlapping sheets exceeds a given maximum length, the stacking wheel can be stopped or rotate at low speed or be slowed down at least so far that all sheets of said group are received completely in a common slot. If it is not ascertained before said group of sheets is fed into the slot that the next sheet or group of sheets follows at a sufficient distance, the stacking wheel is expediently stopped so that the next sheet or group of sheets can also be fed into the same stacker slot. Only when a sufficient distance is determined again, the stacking wheel is positioned for the next slot and one possibly changes over again from individual sheet flow control to synchronized control (synchronization of stacking wheel rotating speed with sheet singling rate).
In case a group of overlapping sheets is determined with a thickness sensor, it is recommendable to stop the stacking wheel since a statement on the total length of the overlapping sheets and consequently a statement on the duration the sheets require for being fed into the slot is not readily possible. The stacking wheel is positioned for the next slot only when a sufficient distance is ascertained between two sheets or groups of sheets again.
However, if the total length of the overlapping sheets is determined and evaluated in addition to the total thickness, one can ascertain exactly at which points the overlap begins and ends. Under these preconditions it is possible to attain a separation of the overlapping sheets by exactly timed brief acceleration of the stacking wheel in such a way that the sheets are fed into separate slots. One must make sure, however, that no excessive accelerations and speeds of the stacking wheel prevent the feed of the sheets or cause them to be thrown out.
For most of the aforementioned embodiments it is expedient to provide a speed sensor for determining the sheet transport speed in order to permit the slots of the stacking wheel to be positioned in time in dependence on the time period remaining for transport of the sheet material to the stacking wheel. In addition, the transport speed can be taken into account when influencing the kinematics of the stacking wheel in such a way that the feed of a sheet or group of sheets into a slot is completed just before the next sheet or group of sheets is fed into the next slot.
Advantageously, the stacking wheel rotates at simple or multiple synchronous speed, synchronous speed vS resulting from nominal singling rate rN (sheets per minute) and number nF of slots per revolution. A multiple synchronous speed means that in nominal operation of the machine, i.e. at synchronized singling rate and stacking wheel speed, not every slot of the stacking wheel receives a sheet. This reduces the risk of consecutive sheets hindering each other during stacking when they hit each other with bends or folds. When a group of sheets is fed at small distances, each slot can then selectively receive a sheet in order to reduce the duration of positioning of the stacking wheel for the next desired slot.
A special embodiment of the invention provides that in addition to or instead of influencing the kinematics of the stacking wheel one influences the sheet speed in at least part of the sheet transport path in order to put irregularly spaced sheets or overlapping sheets at a standardized distance so that the feed of one sheet per slot is more frequently possible. For this purpose a transport system is provided which has at least one transport path segment whose transport speed can be influenced in dependence on the sheet sensor information.
According to another special embodiment of the invention one can influence, in addition to or instead of the kinematics of the stacking wheel, the sheets by means of one or more control fingers, as described fundamentally in DE 27 56 223 C2 mentioned at the outset, the disclosure of which is incorporated herein by reference.